You just never know with Joaquin Phoenix who is going to turn up. At the Venice Film Festival press conference for his new film The Master, he sat on the podium disinterested, inarticulate. It was almost déjà vu, as if the messed-up version of himself that he ‘played’ in his brother-in-law Casey Affleck’s 2010 spoof documentary I’m Still Here had come back to haunt us. Staring at his feet, smoking, he even left the stage briefly – an oddly symbolic gesture for an actor with a love-hate relationship with his job.

Today, the rear of his shirt dripping with sweat, he’s anything but the monosyllabic figure he cut then. ‘I don’t like press conferences,’ he offers. ‘When I go into a room and there are just flashes and a bunch of cameras, I don’t know what to say… it makes me feel sick.’ He even explains his brief departure – a bathroom break. ‘If I’ve got to take a piss, I’m not going to raise my hand and ask permission if I can go,’ he snorts. ‘I’m going to go to the bathroom. I’m sorry if that offends somebody.’

It evidently didn’t offend the Venice jury, who awarded the 38-year-old Phoenix a share of the Best Actor prize with his co-star Philip Seymour Hoffman . Already a two-time Oscar nominee (for his emperor in Gladiator and his stunning turn as Johnny Cash in Walk The Line), this early prize suggests that, after a four-year absence from feature films, he’s now a front-runner in the race for an Academy Award for what is, in The Master, certainly one of the performances of 2012.

Directed by Magnolia’s PT Anderson, as Phoenix’s volatile World War II navy veteran Freddie Quell falls under the spell of Hoffman’s Lancaster Dodd (the founder of a self-help movement known as The Cause), The Master sees the actor at his ferocious best – not least in a scene in jail when he destroys a (real) porcelain toilet in anger. ‘I didn’t plan on it; they just put me in there and that’s what happened,’ he shrugs. ‘I watched all these [YouTube videos of wild] animals being caged up and that seemed to be how they reacted.’

Despite the film’s similarities to Scientology (Dodd is inspired by its founder L Ron Hubbard), Phoenix is not about to stick the knife into the controversial religion followed by stars such as Tom Cruise .

‘I don’t know a lot about religions – I couldn’t even f****** name them. Whatever makes you f****** happy! [If] People find contentment or satisfaction from being Catholic, then that’s awesome. And if they find it with being a Scientologist, then that’s awesome.’

Phoenix’s background bears some relation to the subject of the film. Born in Puerto Rico, his missionary parents moved him and his four siblings around Latin America, famously joining the Children of God cult for a time. But Phoenix is not about to badmouth his upbringing either. ‘What was really amazing about my parents was how they respected us,’ he says. ‘It was never: “Respect your elders but you won’t get respected.” They would run everything by us.’

Still, with his lip curled into a perma-snarl and his blue eyes set to intense, Phoenix gives off the air of a troubled soul – no surprise given he witnessed the death of his older brother River from a drugs overdose outside LA’s Viper Room club in 1993. He’s had his own issues with addiction – he checked himself into rehab to curb his drinking after Walk The Line – and Phoenix is also conflicted about acting in the Hollywood system.Despite his last film being 2008’s acclaimed Two Lovers – for director and friend James Gray , with whom he’s made The Yards and We Own The Night – he announced he was quitting. ‘What I was after more than anything was just to change my approach to acting – because I was bored with it,’ he says. ‘It wasn’t exciting anymore.’

So what did he do? He grew a ridiculous beard, start acting weird on Letterman and performed hip hop. While many speculated Phoenix was going off the rails, when the truth emerged that it was all for I’m Still Here, many felt he and Affleck were biting the hand that feeds. ‘There were some things I felt bad about,’ he admits. ‘[Like] people feeling that we were in some way in trying to make a movie that was attacking them or an industry… I didn’t like that at all. We were really making fun of ourselves.’

If anything, this Sacha Baron Cohen-style immersion offered him a cathartic release from the rigid pressures of making a Hollywood movie. Was he nervous going back into a more ‘traditional’ film afterwards? ‘I’m nervous every time I make a movie,’ he says. ‘And there was part of me that thought: “Oh f***, am I going back into this?”’ But so invigorating did he find The Master, he’s since shot James Gray’s magician tale Nightingale and Spike Jonze’s latest Her. For all his troubles, Joaquin Phoenix is still here.